Ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with energized ions. In semiconductor fabrication, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses, at different energies and different incident angles.
Traditionally, an ion implant process in semiconductor manufacturing is followed by a thermal annealing step to activate implanted dopant atoms. This thermal annealing step controls the spread of dopant atoms beyond the implanted regions in the substrate and hence controls the transition regions in the devices which are critical for device performance. However, as the devices are scaled down to ever smaller dimensions, the thermal budget available for post-implant processes (e.g., annealing) also has to be scaled down, which results in increased sensitivity of device performance to the implanted dopant profiles. Moreover, the implants are typically done on patterned substrates, and aspect ratios for desired implanted regions have become higher, necessitating ever stringent angle control requirements for ion implanters. It is therefore critical to both characterize the ion beam in terms of angle distributions and the effect of substrate topography on implant performance. As an example, if the ions are to be implanted at the bottom of a high aspect ratio trench, due to the intrinsic spread in beam angles, only a fraction of the nominal implanted dose (for a flat surface) will make it to the bottom of the trench. Any small error in the average incident angle may further exacerbate this problem. Therefore, it may be desirable to factor in the effects of device topography and beam characteristics in order to effectively improve overall device performance and yield.
In production ion implanters, an ion beam is typically of a smaller size than a target wafer, which necessitates scanning of either the ion beam across the target wafer or moving the wafer across the ion beam or a combination thereof. Scanning an ion beam typically refers to the movement of the ion beam to increase wafer area that can be implanted, while scanning a wafer typically refers to the relative movement of a wafer through an ion beam. As used hereinafter, “scanning” refers to the relative movement of an ion beam with respect to a wafer or substrate surface. The ion beam is typically either a “ribbon beam” having a rectangular cross section or a “spot beam” having a circular cross section. In the case of a ribbon beam with a dimension longer than the wafer diameter, ion implantation of the wafer may be achieved by keeping the ribbon beam stationary and simultaneously moving the wafer across the ribbon beam in a direction orthogonal to the longer dimension of the ribbon beam. The one-dimensional (1-D) movement of the wafer may cause the ribbon beam to cover the entire wafer surface. In the case of a spot beam, scanning of a wafer may be achieved by sweeping the spot beam back and forth between two endpoints to form a scan path and by simultaneously moving the wafer across the scan path. Sweeping of the spot beam may be accomplished through the use of electrostatic scanners or magnetic scanners wherein the spot beam is controllably deflected from its normal trajectory to span a larger area by changing the electric or magnetic fields respectively in a direction orthogonal to the direction of travel of the spot beam. The strength of the deflection fields determines the total deflection from the normal path of the spot beam, and hence the spot beam may be scanned by changing the field strength of the scanner elements. Alternatively, the spot beam can remain stationary and the wafer can be translated in two orthogonal directions. This mode of operation is typically referred to as a two-dimensional (2-D) mechanical scan. The movement of the wafer across the scan path may be either continuous or incremental.
Any ion beam in an ion implanter may have a distribution of angles which results from, for example, initial ion extraction angles from an ion source and transport of the ion beam through different beamline elements from the ion source to the wafer plane. The distribution of angles in the ion beam typically has a finite angle spread that is not negligible, i.e., not all the ions in the ion beam cross the wafer plane at the same incident angle. As shown in FIG. 1a, a ribbon beam 100 may be considered to comprise a plurality of beamlets 104. These beamlets 104 may have a distribution of angles around a mean direction. Due to beam emittance or divergence, and/or the transport through the beamline elements, these beamlets 104 may strike a wafer surface 102 at different average incident angles. Thus, the wafer surface 102 is exposed to an intrinsic spread of ion beam incident angles. Further, as shown in FIG. 1b, each beamlet 104 may also have an intrinsic spread of incident angles due to, for example, space-charge effects. That is, ions that form the beamlet travel in an average direction but spread out according to a Gaussian-like distribution around the average direction. Since these beamlets form a continuum that describes the ribbon beam, each position in the beam envelope may be considered to have a distribution of angles. It is this angle distribution that is incident on any particular region of the wafer during ion implantation. Similarly, a typical spot beam may also have an intrinsic angle spread such that different portions of the spot beam have a particular ion direction. When the beam or the wafer is scanned in one direction, the resulting incident angle distribution at any point on the wafer is the collective distribution comprising of all the angles in the scan direction.
Ion beam incident angles and intrinsic angle spreads may cause angle variations in the ion implantation process. There are typically three types of angle variations, whose causes and effects are illustrated in FIGS. 2-4 respectively.
FIGS. 2a and 2b illustrate wafer-to-wafer (or inter-wafer) angle variations, wherein wafers 202 and 204 are different wafers separately processed based on the same recipe in the same ion implanter system. Due to small differences in the setups of the ion implanter system and/or beam steering errors, the wafer 202 may be implanted with an ion beam 20 incident at a first angle θ, while wafer 204 may be implanted with an ion beam 22 incident at a second angle θ′, wherein θ′≠θ. θ and θ′ are “angle errors” or “tilt errors” measured with respect to a nominal direction of the wafer surface. In the description that follows, the angle errors are shown as being measured with respect to the normal incidence of the wafer surface. However, in general, such angle errors may be measured with respect to any predetermined angle. The angle errors typically affect all structures or devices on the wafers 202 and 204, and the angle difference can cause wafer-to-wafer variations in device performance. The ion beams 20 and 22 may also have different intrinsic angle spreads which may cause additional doping variations between the two wafers.
FIG. 3 illustrates within-wafer (or intra-wafer) angle variations, wherein different parts of a wafer 302 may experience different ion beam incident angles (θ1, θ2, and θ3, etc.) due to intrinsic angle spread within an ion beam 30. Alternatively, a wafer with an irregular surface (e.g., concave or convex surface) may have significant intra-wafer angle variations even when it is exposed to a perfectly parallel ion beam (i.e., an ion beam with zero angle spread). Such intra-wafer angle variations can cause significant performance variations for devices located in different parts of the same wafer.
FIG. 4 illustrates device-level angle variations. As shown, a first part of an ion beam (40) may have a first average incident angle, and a second part of the ion beam (42) may have a second average incident angle, which may cause a trench 402 or a mesa 404 to see a spread of incident angles. As a result, the bottom of the trench 402 may have a different dopant profile from its sidewalls. And each sidewall of the trench 402 may have a different dopant profile from another. Similarly, the mesa 404 may have one side more heavily doped than an opposite side. For certain applications, such an asymmetrical dopant profile may not be acceptable.
If the ion beam incident angles and/or angle spread are not properly controlled in the implantation and doping processes, the above-described angle variations may cause device performance variations and even yield problems, as illustrated in FIGS. 5a and 5b. FIG. 5a depicts an ion beam 50 with zero angle error and a small angle spread. The ion beam 50 is used to dope a substrate surface 502, a part of which is masked by a structure 504 (e.g., a gate stack) having vertical sidewalls. Since the ion beam 50 is aligned with the sidewalls, the resulting dopant profiles 52 and 54 on either side of the structure 504 are symmetric. Even with the symmetric dopant profiles, effective ion doses in the substrate 504 are not uniform. Due to the shadowing effect from the structure 504, the effective ion doses close to the edges of the structure 504 are lower than areas far away from the edges of the structure 504.
If, however, the ion beam 50 has a small angle error as shown in FIG. 5b, the shadowing effect from the structure 504 can cause the resulting dopant profiles 56 and 58 to be asymmetrical which causes a degradation in the IC performance. In an extreme case as for high aspect ratio structures, the asymmetry in the dose distributions may cause the device to be inoperable. The difference in effective ion doses on either side of the structure 504 is referred to as a “dose skew.” The difference in effective ion incident angles on either side of the structure 504 is referred to as an “angle skew.” Such dose skews or angle skews may cause asymmetry in physical features of a device as well as in device performance. For example, if the structure 504 is a gate stack of a metal-oxide-semiconductor (MOS) transistor, the source and drain regions 503 on either side of the structure 504 may effectively be shifted due to the angle skew, such that one region may have no overlap with the gate while the other region may have too much overlap with the gate. The performance asymmetry (or performance skew) may be exhibited, for example, in a number of transistor parameters such as contact resistance (Rc), series resistance (Rsd), threshold voltage (Vt), overlap capacitance (Cov), on current (Id,sat), off current (Ioff), and gate leakage (Ig,off). That is, a transistor originally designed to be symmetric may become lopsided, wherein, if the source and drain contacts are switched, one or more of these transistor parameters may change significantly.
Note that the structure 504 may be just one of the devices one the substrate 502 whose topography makes it sensitive to ion beam angle variations (e.g., beam steering errors and angle spreads). Typical ion implants that are sensitive to angle variations include, but are not limited to, contact plug implants, source-drain extension implants, and halo implants. In addition, different type of structures or devices (with different orientations) may be sensitive to angle variations in different directions. If the angle error and/or angle spread of the ion beam 50 are not properly controlled, similar yet varying effects may be seen across different parts of the substrate 502 or across different wafers. As the device feature size continues to shrink, the device-level, wafer-level, and wafer-to-wafer angle variations, if left uncontrolled, may cause more performance variations and other detrimental effects.
The setup-to-setup variations in ion beam angle distributions may also cause process repeatability problems in an ion implanter system. In extreme cases, the ion beam angle variations may lead to yield losses. As described above, uncontrolled ion beam incident angles and angle spread may cause significant performance variations among different wafers processed in the same implanter. Existing methods for setting up an ion implanter system have typically been limited to the repeatability of implant doses. No known method manages to effectively control or minimize dose skews and/or angle skews over an entire wafer and/or among different wafers.
In view of the foregoing, it would be desirable to provide a technique for improving ion implantation which overcomes the above-described inadequacies and shortcomings.